The present invention relates to a photosensitive device implementing electric charge coupling and accumulation, and enabling light signals to be analyzed. A major although non-exclusive application lies in so-called xe2x80x9clidarxe2x80x9d apparatuses for measuring the range to obstacles or to media that reflect or back-scatter short pulses of light. Such lidars are used in particular to determine the altitude and the position of clouds by measuring the travel time of laser pulses back-scattered by the surfaces of clouds. A particular type of lidar known as a xe2x80x9cwindxe2x80x9d lidar serves to measure average wind speed in various altitude ranges by using the wavelength shift that is caused by the Doppler effect.
Proposals have already been made (document FR-A-2,735,935) for a photosensitive device implementing charge coupling and storage for analyzing light signals and incorporating a photodetector matrix. The detector is constituted by a charge-coupled device (CCD) and presents numerous advantages, particularly when high sensitivity is required. CCD detectors can be made so as to have high quantum efficiency from the ultraviolet to the near infrared; they make it possible on a single chip to combine functions of integrating charge and of performing analog storage. They enable charge to be transferred at high frequency. Their read noise is low.
For a wind lidar, the device is associated with a spectrometer enabling each column of the image zone to be associated with a different spectrum shift, and thus with a different speed.
Improvements to the above device are described in French application FR-A-2,769,450 corresponding to EP 0 907 284 and U.S. Pat. No. 6,084,659 to which reference can be made; they enable electrical charges to be accumulated in charge coupling means, thereby enabling the device to be read at intervals corresponding to a plurality of successive observation periods, i.e. only after several successive laser shots when the device is incorporated in a lidar.
French patent FR-A-2,769,450 discloses a photosensitive device implementing charge coupling and accumulation for analyzing a light signal and comprising:
an image zone constituted by a matrix of M rows by N columns of photosensitive sites for receiving the light signal during successive acquisition periods;
a first memory zone constituted by a matrix of P rows and N columns of non-photosensitive sites for receiving in each site of a first row constituting an integration row, the total charge in a respective column of the image zone, at the end of each successive acquisition period;
a read register having a single row of N sites for receiving in parallel the charge stored in the last row of the first memory zone;
a second memory zone of P rows and N columns for receiving in parallel the charge admitted to the N sites of the read register and for applying it to N sites of a single line in the image zone or of the first memory zone, thereby accumulating in each row of the first memory zone the charge as accumulated over an observation period and coming from a plurality of successive summing operations; and
clocking means operating:
at the end of each acquisition period and in a time that is short relative to the acquisition period, to transfer the charge stored in all of the sites in each column of the image zone along the column to a respective site of an integration line in which charge is summed;
during each observation period, to transfer the charge accumulated in each site of the integration line stepwise at the acquisition rate, along the columns of the memory zone towards the read register; and
to read the read register after a plurality of observation periods.
This makes it possible, in particular, to loop back to the image zone or the first memory zone, so that each row of the first memory zone accumulates charge coming from a plurality of successive summing operations.
In a first embodiment, illustrated in FIG. 2, the read register has at least N additional sites to which charge coming from the last row of the first memory zone is transferred in order to be reintroduced into the second memory zone, and the second memory zone is designed to enable the accumulated charge it contains to be transferred serially to an integration zone. At the end of each acquisition period, the contents of the lines corresponding to the second memory zone and the image zone are accumulated in said integration zone. The integration line is constituted by a shift register having at least 2N sites. Among the N sites, there are N sites receiving in parallel the content of the last line of the second memory zone, and N sites capable of summing the charge they contain with charge transferred from the memory zone.
That solution has the advantage of avoiding high levels of charge on a given row disturbing lower levels of charge present in other rows. However, implementing it gives rise to difficulties in implanting sequencing clock electrodes for the N sites which receive the charge transferred from the image zone.
In a second case disclosed in FIG. 3 of FR-A-2,769,450, the second memory zone is designed so that the charge contained in each row of said second memory zone and coming from corresponding rows of the first memory zone can be transferred into the corresponding rows of the image zone prior to each observation period.
That requires only the provision of clocking having additional outputs for controlling transfers to and from the auxiliary second memory.
Implementation remains simple. However charge is transferred back and forth through the image zone. Imperfections in transfer between rows mean that in some applications time samples of low flux are polluted by the row that is the most illuminated, and consequently that charge becomes xe2x80x9csmearedxe2x80x9d.
The invention seeks in particular to provide a device that avoids the above limitations to a large extent.
To this end, the invention provides a device comprising:
an image zone constituted by a matrix of M rows and N columns of photosensitive sites for receiving a light signal during successive acquisition periods;
a transfer zone constituted by a matrix of P rows and N columns of non-photosensitive sites for receiving in each site of a first row constituting an integration row the total charge in a respective column of the image zone at the end of each successive sampling period;
a storage zone of P rows and N columns in which each column is adjacent to a respective column of the transfer zone and each site is designed to receive the charge contained in an adjacent site of the transfer zone and to accumulate it with the charge it contains already; and
clocking means operating:
(a) during each of R successive observation periods, to obtain P successive time samples of duration T, each by simultaneously transferring charge into the sites of the memory zones and, in a period that is short relative to the duration T, subsequently transferring the charge from all of the sites in a single column into the first row of the transfer zone after moving charge from each row of the transfer zone to the following row; and after said P samples have been taken, to transfer the charge of each row of the transfer zone to the adjacent site of the storage zone; and
(b) after R observation periods, to cause the storage zone to be read row by row.
It should also be observed that the device finds an additional application in constituting a fast low-flux radiometer using the image and memory zones of the device as a receiver of low light levels.
The above characteristics and others will appear more clearly on reading the following description of a particular embodiment given by way of a non-limiting example. The description refers to the accompanying drawings.